1. Field of the Invention
This invention relates to flame-retardant polyester resin compositions. More particularly, it relates to a flame-retardant polyester resin composition having excellent mechanical properties and thermal resistance and also exhibiting good releasability of the molded article and rapid feed of resin chips into the molding machine at the time of molding.
2. Description of the Prior Art
Polyethylene terephthalate resins reinforced with glass fibers are being widely used as engineering plastics because of their excellent resin properties such as mechanical properties, thermal resistance, chemical resistance and moldability.
However, similar to other synthetic resins, such glass-fiber reinforced polyester resins involve the problem of flammability, it is of great importance to impart flame retardancy to them.
Since glass-fiber reinforced polyethylene terephthalate resins are usually molded at a relatively high temperature of 250.degree. to 300.degree. C., the thermal resistance of the flame retarder added thereto is important. Thus, it is necessary to use a flame retarder which barely decomposes even at elevated temperatures and, moreover, causes neither discoloration of the finally obtained molded articles nor degradation of the resin properties. Since glass-fiber reinforced polyethylene terephthalate resins have high heat distortion temperatures, they are being widely used as materials for the manufacture of products (such as electrical parts) for use in high-temperature environments. Accordingly, in imparting flame retardancy to such glass-fiber reinforced polyester resins, it is important that they not only have excellent initial properties such as flame retardancy, mechanical strength and thermal resistance, but also show no degradation of the flame retardancy due to the thermal escape of the flame retarder even after they have been utilized under a high-temperature atmosphere for a long period of time. In other words, thermal stability of the molded articles is required.
Since a flame retarder (C) as described later is excellent in, for example, thermal resistance and flame-retardant properties, the present inventors attempted to incorporate a combination of this flame retarder (C) and antimony trioxide into a glass-fiber reinforced polyethylene terephthalate resin and thereby improve its flame retardancy. When the flame retarder (C) was added in small amounts, the resulting composition had excellent performance as a result of improvement not only in flame retardancy but also in mechanical properties. However, when the flame retarder (C) was incorporated with a view to achieving a higher degree of flame retardancy, the resulting composition showed a decrease in flow properties and hence moldability under certain molding conditions (for example, when the injection molding temperature was raised or when the residence time was prolonged).
Furthermore, it is very important from the viewpoint of economy to shorten the time elapsed until the molded article is taken out of the mold, or the molding cycle. In practice, this is usually accomplished by shortening the injection time and the cooling time as much as possible. However, since the cooling time depends largely on the size and thickness of the molded article, the molding cycle is practically governed by the time required for feeding a predetermined amount of chips, termed the feed time. Accordingly, the molding cycle can be expected to become shorter if the feed time is reduced.